Traditionally, an antenna for receiving satellite signals is generally designed as a three-dimensional helical structure, which is a three-dimensional antenna having its radial metal plates extended in a helical course along a coaxial line to define a three-dimensional space. In Great Britain Pat. No. 2,258,776, a three-dimensional antenna structure is disclosed. In that patent, a plurality of radial metal plates is arranged around a coaxial line and extended in a helical course to define a three-dimensional multiple-filar helical antenna structure. Since such three-dimensional antenna has a better capability of receiving circularly polarized signals coming directly from above, therefore this kind of antenna is usually used for a global positioning system (GPS) to receive a coordinates positioning signal from a satellite group. Further, this kind of three-dimensional antenna is suitable to serve as an omni-directional antenna for receiving vertically and horizontally polarized signals. However, the shortcomings of this three-dimensional antenna resides on that its structure is not strong enough for certain applications, and corrections cannot be made easily without affecting its overall performance.
In view of the foregoing shortcomings of the multiple-filar helical antenna, many antennas for receiving satellite signals in poor weathers adopt a patch antenna instead. For example, the antenna installed at the outside of an airplane is a patch antenna which has a radial metal plate attached onto an insulator on an airplane. However, this kind of patch antenna only has a low gain when the airplane is rising at a small angle. To overcoming this drawback, antenna designers install a plurality of patch antennas onto different positions and at different angles of the body of an airplane, so that a feeder end of each patch antenna is connected to the same receiver to receive satellite signals. Since it takes a number of patch antennas to achieve this result and it is difficult to integrate all signals received by the patch antennas, therefore the cost becomes very high.
To solve the foregoing problem, U.S. Pat. Nos. 6,369,776, 6,424,316, and 6,552,693 by Leisten were disclosed, wherein these patents effectively reduce the size of traditional quadri-filar antennas and design a novel quadri-filar antenna structure. Referring to FIG. 1, the antenna has a cylindrical body 12 made of a ceramic material, and four antenna elements 10A, 10B, 10C, 10D disposed on a circumferential surface at an end proximate to the cylindrical body 12 and extended along its axial direction in a helical course, and each antenna element 10A, 10B, 10C, 10D is a metal sheet, and the cylindrical body 12 includes a penetrating hole 14 disposed along the axial direction of the center, and a metallic lining 16 is covered on the internal wall of the penetrating hole 14 and an insulator 17 is installed therein. The insulator 17 at its center installs an axial feeder conductor 18, and the axial feeder conductor 18 and the metallic lining 16 form a feeder structure, such that a feeder line of a signal receiver (not shown in the figure) can be connected to the antenna elements 10A, 10B, 10C, 10D through the feeder structure. The antenna structure further comprises a plurality of radial antenna elements 10AR, 10BR, 10CR, 10DR disposed on a distal surface of the cylindrical body 12, and each radial antenna element 10AR,10BR,10CR, 10DR is also a metal sheet coupled with the corresponding end of the antenna element 10A, 10B, 10C, 10D respectively, such that an end of the antenna element 10A, 10B, 10C, 10D is coupled to the feeder structure respectively, and a common grounding conductor 20 is disposed on the circumferential surface at the other end proximate to the cylindrical body 12, and the common grounding conductor 20 is embedded into the circumferential surface at the other end of the cylindrical body 12, and an end of the common grounding conductor 20 is coupled to another end of the antenna elements 10A, 10B, 10C, 10D, and the other end is extended to the other distal surface of the cylindrical body 12 to form a “Sleeve Balun” coupled to the metallic lining 16. Referring to FIG. 1 for the antenna structure, each antenna element 10A, 10B, 10C, 10D has a different length and a different shape, wherein the two antenna elements 10B, 10D are extended in a meandering course along the circumferential surface 12D of the cylindrical body 12 in a helical form. Therefore, its length is longer than the two antenna elements 10A, 10C in other linear courses extended in a helical course along the circumferential surface of the cylindrical body 12.
From the literature published by Leisten, the quadri-filar antenna uses a ceramic material with a high dielectric constant (εr=36) as a base, and the four antenna elements 10A, 10B, 10C, 10D have an electric length of half a coil and a half wavelength. Therefore, the size of traditional quadri-filar antennas can be reduced greatly. However, the manufacturing process is more complicated and has to go through the copper plating, exposure, etching and laser trimming processes. Particularly, the height of the Sleeve Balun must be controlled within several micrometers to eliminate unbalanced currents and thus greatly increasing the manufacturing hours, manpower and costs.
Further, manufacturers simplify the foregoing processes by Leisten's patented inventions by designing a coaxial cable precisely embedded into the penetrating hole 14. To meet the required impedance, a metal shielding layer of the metallic lining 16 on the coaxial cable is manufactured according to a particular specification. Therefore, a coaxial cable of a length of several centimeters costs about 3˜4 US dollars. Furthermore, for antennas of different specifications and requirements, it is necessary to redesign a different coaxial cable, and thus it is not easy to perform impedance matching and adjustment for antennas, and the manufacturing cost cannot be lowered effectively due to the expensive cost of the coaxial cable.